1. Field of the Invention
The present invention is directed to a method for connecting communication terminals to a switching system.
2. Description of the Related Art
In modern communication technology, there exists an ever-increasing demand for broadband transmission of information such as, still and moving pictures in video telephone applications and of large volumes of data on the Internet. As a result, the significance of transmission technologies for high or variable data transmission rates (greater than 100 MBit/s) which take into account both the requirements of the data transmission (high speed at variable transmission bit rate) and the requirements of voice data transmission (maintenance of time correlations in the case of a data transmission by a communication network) is increasing. The asynchronous transfer mode (ATM) is a known data transmission method for high data speeds which additionally meets the requirements of voice data transmission.
This requires, in particular, broadband data transmission right up to the communication end point area, i.e. from the transmitting communication terminal to the receiving communication terminal that is frequently called end-to-end transmission in the literature. The consequence is that the number of ATM-capable communication terminals, i.e. of communication terminals which support the ATM data format for a transmission of data between communication terminals and the switching system associated with the communication terminal, rises drastically.
If such ATM-capable communication terminals are connected to a switching system not directly but via a non-ATM-oriented communication network, (for example due to a large distance between the communication terminal and the switching system associated with the communication terminal) the ATM data format must be converted to the data format of the communication network before any transmission of data via the communication network takes place. If this communication network is a data network which frequently already exists in companies and in which, preferably, Internet protocols (IPs) are used such as, for example, the “Ethernet” or the “Tokenring”, the transmission of data to be transmitted in a voice call will take place via such an IP-oriented communication network in accordance with the Realtime Transport Protocol (RTP) according to ITU-T (International Telecommunication Union) Standard H.225.0.
If compressed voice data are transmitted—as used, for example, in mobile radio—these compressed voice data must be decompressed at the transmitter end before they are transmitted via the IP-oriented communication network, converted into the IP data format according to the RTP protocol and then recompressed for the transmission. Furthermore, the data must be decompressed at the receiver end, converted into the original data format and then recompressed for the further transmission. This frequent compression/decompression of the voice data leads to a corruption at the receiver end of the voice data originally transmitted which may be audible and can thus be sensed to be disturbing.
It is the object of the following invention to specify a method by means of which voice data transmission via an IP-oriented computer network is made possible without loss of voice quality.
To provide a better understanding of a data transmission between an ATM-capable communication terminal and a switching system associated with the communication terminal, it appears to be necessary first of all to discuss known principles again in greater detail.
A data transmission between an ATM-capable communication terminal and a switching system associated with the communication terminal—frequently called Home PBX of the communication terminal in the literature—usually takes place on the basis of so-called CPS (Common Part Sublayer) packets—referred to as substructural elements SE in the following text—according to the so-called ATM adaptation layer AAL-Typ2. In this arrangement, the ATM adaptation layer AAL provides an adaptation between the format of the ATM layer (layer 2) and the network layer (layer 3) of the OSI (Open System Interconnection) reference model.
A substructural element SE is composed of a 3-byte-long cell header SH and a payload area I of variable length (0 to 64 bytes). The cell header of a substructural element SE, in turn, is subdivided into an 8-bit-long channel identifier CID, a 6-bit-long length indicator LI, a 5-bit-long user-to-user indication UUI and a 5-bit-long cell header error control HEC.